Type systems, complexity analysis, and assembly language programming, for example, aren't "hardcore disciplines in computing". They're the basic foundation upon which the rest of our knowledge is built.

Your second sentence pretty much defines "hardcore". The "hard core" of anything is what supports the rest. Originally the term is from the mid-19th century and refers to a layer of broken stones and bricks that provided support for a building project, commonly roads.

http://boards.straightdope.com/sdmb/showthread.php?t=523412